fix(allwinner): check RSB availability in DT on H6

fix(allwinner): check RSB availability in DT on H6

At the moment we access the RSB bus on all Allwinner H6 boards
unconditionally, even though some boards do not have any PMIC at all,
while others have some I2C devices connected to the same pins.
The latter case is just fragile, but the first case leads to a hang on
at least one board, as reported by Jernej.

Scan the devicetree, to check for the availability of the RSB bus node.
Proceed only if the RSB DT node is actually enabled.

Change-Id: Iea7dcfe3e085e173334d098ec4ddcb6c4b085771
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reported-by: Jernej Skrabec <jernej.skrabec@gmail.com>